Ion exchange membrane fuel cell power plant with water management pressure differentials

ABSTRACT

A proton exchange membrane fuel cell device with an internal water management and transfer system includes a plurality of adjacently arranged proton exchange membrane assemblies including a proton exchange membrane component; a pair of porous anode and cathode catalyst layers situated on either side of the proton exchange membrane; and porous plate assemblies interposed between and in contact with each of the adjacent proton exchange membrane assemblies. Oxidant gas is supplied to oxidant gas supply channels, and fuel gas to fuel gas supply channels formed in the porous plate assemblies for distribution to the cathode and anode catalyst layers, respectively. A water coolant circulating system is formed in each of the porous plate assemblies and causes each of the porous plate assemblies to become saturated with coolant water. The reactant flow fields are pressurized to a pressure which exceeds the coolant water circulating pressure by a selected ΔP so as to ensure that product water formed on the cathode side of each membrane assembly will be pumped through the porous plates into the coolant water flow field and become entrained in the circulating coolant water stream.

TECHNICAL FIELD

The present invention relates to ion exchange electrolyte membrane fuelcell power plants and more particularly to a water management systemtherefor.

BACKGROUND ART

Fuel cell power plants employing ion-exchange electrolyte membranesconfined between respective porous cathode and anode electrode reactantflow field plates are known in the art. Operation of such power plantswith hydrogen as the fuel reactant and oxygen or air as the oxidantreactant results in the production of product water at the cathode sideof each cell in the power plant. Furthermore, as hydrogen ions travelthrough the membrane, they drag water through the membrane from theanode side to the cathode side. This results in two problems that mustbe addressed. The first problem is dryout of anode side of the membrane,and the second problem is flooding of the cathode side of the membrane.Either of these problems, if not solved, can result in malfunction ofthe power plant.

One solution to both of the aforesaid problems is disclosed in AustrianPatent No. 389,020 which describes an ion-exchange membrane fuel cellstack that utilizes a fine pore water coolant plate assemblage toprovide passive cooling and water management control in the cells in apower plant. The Austrian system utilizes a water-saturated fine poreplate assemblage between the cathode side of one cell and the anode sideof the adjacent cell to both cool the cells and to prevent reactantcross-over between adjacent cells. The fine pore plate assemblage isalso used to move product water away from the cathode side of theion-exchange membrane and into the coolant water stream; and to movecoolant water toward the anode side of the ion-exchange membrane toprevent anode dryout. The preferred directional movement of the productand coolant water is accomplished by forming the water coolant plateassemblage in two parts, one part having a pore size which will ensurethat product water formed on the cathode side will be wicked into thefine pore plate and moved by capillarity toward the water coolantpassage network which is inside of the coolant plate assemblage. Thecoolant plate assemblage also includes a second plate which has a finerpore structure than the first plate, and which is operable to wick waterout of the water coolant passages and move that water toward the anodeby capillarity. The fine pore and finer pore plates in each assemblageare grooved to form the coolant passage network, and are disposed inface-to-face alignment between adjacent cells. The finer pore plate isthinner than the fine pore plate so as to position the water coolantpassages in closer proximity with the anodes than with the cathodes. Theaforesaid solution to water management and cell cooling in ion-exchangemembrane fuel cell power plants is difficult to achieve due to thequality control requirements of the fine and finer pore plates, and isalso expensive because the plate components are not uniform in thicknessand pore size.

It would be desirable to provide a water management system for use in anion exchange membrane fuel cell power plant that can operate efficientlywith fine pore water wicking plates, such as are disclosed in theaforesaid Austrian patent, but which plates are all essentially uniformin thickness and pore size.

DISCLOSURE OF THE INVENTION

This invention relates to an ion exchange membrane fuel cell power planthaving an internal coolant and product water management system. Each ofthe cells in the power plant includes an ion exchange membraneelectrolyte component. Fine pore plate assemblies are interposedbetween, and in contact with, adjacent membrane components. Each finepore plate assembly contacts the anode catalyst layer on one of theadjacent membrane components, and contacts the cathode catalyst layer onthe next adjacent membrane component. The fine pore plate assembliesinclude a plurality of oxidant gas supply channels opening onto thecathode catalyst layers, and a plurality of fuel gas supply channelsopening onto the anode catalyst layers. The fine pore plate assembliesalso include internal coolant water circulating channels which provideseparate coolant water flow fields between adjacent cells in the powerplant. Coolant water from the coolant water flow fields is wicked intothe fine pore plates by capillarity so as to completely fill the finepore plates with water thereby preventing cell-to-cell oxidant/fuel gascrossover between adjacent cells in the power plant, and also providingcoolant water on both sides of each of the electrolyte membranes.

The oxidant reactant gas is pressurized to a pressure which exceeds thecoolant loop pressure by a predetermined ΔP. Thus the oxidant reactant(cathode) flow fields will be maintained at a pressure which exceeds thepressure in the coolant water flow fields. The resultant ΔP will causeproduct water appearing on the cathode side of each membrane to bepumped through the fine pore plates into the coolant water flow field.The desired pressure differential can be maintained with a system ofvalves and regulators which may be manually or automatically operated.The fuel reactant (anode) flow field pressure will be maintained at alevel which allows coolant water migration from the coolant loop throughthe fine pore plates toward the membrane, but which prevents flooding ofthe anode surface of the membranes with coolant water. Thus, productwater will be removed from the cathode side of each membrane, andcoolant water will be delivered to the anode side of each membrane. Thewater management system of this invention depends on positive pressuredifferentials being established and maintained between the cathodeoxidant reactant flow fields and the water coolant flow fields, andtherefore allows uniform fine pore plates to be used in power plantswhich are constructed in accordance with this invention.

It is therefore an object of this invention to provide a proton exchangemembrane fuel cell assembly having a water flow management system thatdoes not require the use of structurally differentiated fine pore platecomponents.

It is a further object of this invention to provide a fuel cell assemblyof the character described which ensures the removal of product waterfrom the cathode side of each cell in the power plant.

It is yet another object of this invention to provide a fuel cellassembly of the character described which utilizes a higher fluidpressure in the oxidant reactant gas flow field and a lower fluidpressure in the water coolant flow field in order to establish aninternal pressure differential that pumps product water from the cathodeside of each cell in the power plant.

These and other objects and advantages of the invention will become morereadily apparent from the following detailed description of theinvention when taken in conjunction with the accompanying drawings inwhich:

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a somewhat schematic cross sectional view of a proton exchangemembrane fuel cell power plant formed in accordance with this invention;

FIG. 2 is a front elevational view, partially broken away, of one of thefine pore plate water coolant separator plate assemblies formed inaccordance with this invention;

FIG. 3 is a sectional view taken along line 3--3 of FIG. 1; and

FIG. 4 is a schematic view of an operating system employed in the powerplant which will provide the necessary pressure differential between theoxidant reactant flow field and the water coolant flow field.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to FIGS. 1 and 2, there is shown a fuel cell power plantwhich is designated generally by the numeral 10. The power plant 10includes a plurality of ion exchange membrane fuel cell assemblies 11,an oxidant reactant gas inlet manifold 12, an oxidant gas outletmanifold 13, a fuel gas inlet manifold 14, and an a fuel gas outletmanifold 15. In the power plant orientation shown in the drawings, thefuel outlet manifold 15 serves as a sump for collecting coolant andproduct water from the fuel cells 11. A cathode catalyst layer 22 isdisposed on one surface of the ion exchange membrane 21.

Coolant water flow fields are formed in juxtaposed fine pore plates 27and 28 which are interposed between adjacent cells in the power plant.Coolant water is supplied to the cells through main distributionchannels 32 and thence through a multitude of coolant channels 31 whichare formed in the plates 27 and 28. Coolant water is withdrawn from thewater sump via a conduit 33 by a pump 34 and is directed into a conduit36 which leads to a water inlet port 35 that communicates with the maindistribution channel 32. As coolant water flows through the channels 31,it penetrates and fills the pores of the plates 27 and 28 so as toprevent reactant gas cross-over between adjacent cells in the powerplant.

Referring now to FIG. 3, further details of the assemblies 11 are shown.FIG. 3 shows portions of two adjacent fuel cells 20 and 20' of theassembly 11. It will be noted that, in addition to the cathode catalystlayer 22, the membrane 21 of the fuel cell 20 is provided with an anodecatalyst layer 23. Respective substrates 24 and 25 contact the catalystlayers 22 and 23. Porous hydrophilic plate assemblies 26 and 26' aresituated between adjacent cells in the power plant. The plate assemblies26 are formed from separate plates 27 and 28 that are substantiallyidentical in composition, pore size, and structure. The plate assemblies26 provide oxidant and fuel gas channels 29, 29', and 30, 30',respectively.

Referring now to FIG. 4, there is shown schematically a coolant andreactant pressure control system which can be used to ensure that fueland oxidant reactant pressures are properly maintained relative to thecoolant water system pressure so as to ensure proper cooling and watermanagement in accordance with the system and method of this invention.In FIG. 4, the cell membrane is denoted by the numeral 21; the cathodeoxidant reactant flow field is denoted by the numeral 29; the anode fuelreactant flow field is denoted by the numeral 30; and the water coolantflow field is denoted by the numerals 31, 32, as in the previousfigures. The coolant water circulates through a coolant loop 36 which ispressurized by a pump 34. The pump 34 establishes a predeterminedcoolant water pressure in the loop 36, which pressure may be furtherregulated by a variable valve 40 prior to entering the coolant flowfield 31, 32. If the pump 34 is a fixed rate pump, the valve 40 will beuseful for varying coolant pressure in the event that pressureadjustments are necessary. If a variable speed pump were used, the valve40 could possible be eliminated from the system. A pressure transducer42 is disposed downstream of the pump 34 and valve 40, and is operableto measure the pressure of the coolant water stream before it enters thecoolant flow field 31, 32. The pressure transducer 42, the valve 40 andthe pump 34 may be connected to a power plant microprocessor controller44 via lines 46, 48 and 50. Coolant pressure input from the pressuretransducer 42 will cause the controller 44 to regulate the pump 34and/or valve 40 when necessary to achieve a target coolant pressure.

The oxidant reactant may be essentially pure oxygen derived from apressurized oxygen container, or may be air which is pressurized by acompressor or air blower. The oxidant reactant is delivered to thecathode flow field 29 through a line 52. The line 52 may contain avariable pressure regulating valve 54 and a downstream pressuretransducer 56 which measures the extant pressure of the oxidant streamas it enters the cathode flow field 31, 32. The pressure transducer 56is connected to the system controller 44 via line 53 and the variablevalve 54 is connected to the controller 44 by line 60. When a variablecompressor or pump 62 is used to pressure an air oxidant, appropriateconnections may be made with the controller 44. The controller 44 canthus make appropriate corrections in the oxidant reactant pressure whensystem operating conditions so dictate by varying the valve 54 or thepump/compressor 62.

The fuel reactant is fed into the anode flow field 30 by means of a line64. The fuel reactant gas will typically be contained in a pressurizedcontainer, or in a pressurized fuel conditioning or reforming system(not shown). A variable valve 66 is operable to regulate the pressure ofthe fuel reactant as it enters the anode flow field 30. The fuelreactant pressure is monitored by a pressure transducer 68 which isconnected to the system controller 44 by a line 70. The variable valve66 is connected to the system controller 44 by a line 72. General systemoperating conditions will require that the oxidant gas pressure in thecathode flow field 29 exceed the coolant pressure in the coolant flowfield 31, 32 by a predetermined ΔP so as to ensure movement of theproduct water from the cathode side of the membrane 21 toward thecoolant flow field 31, 32. Likewise, the fuel gas reactant pressure inthe anode flow field should also exceed the coolant water pressure by asecond predetermined ΔP which will allow appropriate migration of watertoward the anode side of the membrane 21 to prevent membrane dryout, butwill not allow a degree of water migration that would flood the anodesurface of the membrane 21.

The valves, pumps, and the like equipment for producing the desiredpressure differentials may be manually operable, or may be automaticdevices that can be operated by a system microprocessor controller. TheΔP's needed to properly operate the power plant may vary depending onpower output, plant size, internal pressures of reactants, and the like.Pressure gauges can be used in place of pressure transducers, ifdesired.

Since many changes and variations of the disclosed embodiment of theinvention may be made without departing from the inventive concept, itis not intended to limit the invention otherwise than as required by theappended claims.

What is claimed is:
 1. A method for operating a solid polymerelectrolyte membrane fuel cell power plant, said method comprising thesteps ofa) providing oxidant and fuel reactant gas streams on oppositecathode and anode sides of the solid polymer electrolyte membrane; b)providing a circulating water coolant stream on said cathode side of theelectrolyte membrane; c) providing a fine pore plate between saidoxidant gas stream and said circulating water coolant stream; and d)pressurizing said oxidant reactant gas stream to a first predeterminedpressure; e) pressurizing said water coolant stream to a secondpredetermined pressure which is less than said first predeterminedpressure so as to create a positive target pressure differential (ΔP)between said oxidant reactant gas stream and said water coolant stream,which ΔP is operative to pump product water formed on the cathode sideof the electrolyte membrane through said fine pore plate and into saidcirculating water coolant stream.
 2. The method of claim 1 wherein saidoxidant reactant gas stream is pressurized to a pressure which is aboveambient pressure.
 3. The method of claim 1 further comprising the stepof periodically releasing product water from said water coolant stream.4. A solid polymer electrolyte membrane fuel cell power plant assemblycomprising:a) oxidant and fuel reactant gas streams on opposite cathodeand anode sides of the solid polymer electrolyte membrane; b) acirculating water coolant stream on said cathode side of the electrolytemembrane; c) a fine pore plate between said oxidant gas stream and saidcirculating water coolant stream; d) means for pressurizing said oxidantreactant gas stream to a first predetermined pressure; and e) means forpressurizing said water coolant stream to a second predeterminedpressure which is less than said first predetermined pressure so as tocreate a positive target pressure differential (ΔP) between said oxidantreactant gas stream and said water coolant stream, which ΔP is operativeto pump product water formed on the cathode side of the electrolytemembrane through said fine pore plate and into said circulating watercoolant stream.
 5. The assembly of claim 4 comprising means forpressurizing said oxidant reactant gas stream to a pressure which isabove ambient pressure.
 6. The assembly of claim 4 further comprisingmeans for periodically releasing product water from said water coolantstream.